Nutrient and mercury variations in soils from family farms of the Tapajós region (Brazilian...

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Agriculture, Ecosystems and Environment 120 (2007) 449–462

Nutrient and mercury variations in soils from family

farms of the Tapajos region (Brazilian Amazon):

Recommendations for better farming

N. Farella a, R. Davidson a,b,*, M. Lucotte a, S. Daigle c

a Institut des sciences de l’environnement, Universite du Quebec a Montreal, CP 8888 Succ, Centre-Ville, Montreal, Quebec, Canada H3C 3P8b Biodome de Montreal, 4777 Pierre De Coubertin, Montreal, Quebec, Canada H1V 1B3

c Institut de recherche en biologie vegetale, 4101 Sherbrooke est, Montreal, Quebec, Canada H1X 2B2

Received 30 January 2006; received in revised form 20 October 2006; accepted 2 November 2006

Available online 21 December 2006

Abstract

In the Brazilian Amazon, colonization is modifying the landscape at an exceedingly fast pace. Recently established households practice

slash-and-burn agriculture and participate in the overall deforestation of the Amazon. Near the Transamazon highway, these family

agricultural practices are the main cause of deforestation. The study presented here is oriented toward a better understanding of the impacts of

farming practices on soil chemical composition. This study used a sampling design based on soil samples taken on farm plots, which had been

submitted to a wide range of spatial and temporal sequential land-uses, including soils that were only recently denuded. The data shows that

soil responses (organic matter (OM) content, fertility and mercury (Hg) retention) to these varied land-uses were relatively similar, suggesting

that the most important event determining the responses was deforestation itself. This is well illustrated by the Hg content of soils, which

changed immediately after deforestation and then only slightly thereafter. This phenomenon could also be seen in the base cation (calcium

(Ca), potassium (K) and magnesium (Mg)) content which rose drastically after deforestation and tended to stay high for a period up to 10 years

of cropping and pasture. This lasting cation rise is reflected by ammonium (NH4) displacement from surface soils. Indeed, inorganic nitrogen

(N) is the most important nutrient loss upon deforestation. Nonetheless, when time spent in fallow was greater than 15 years, base cations (Ca,

Mg, K), available N and phosphorus (P) contents tended to go back to initial forest soil values and in some cases to exceed them. Soil type was

seen to mediate responses to land-use. Clay-sandy soils showed a lower content of available N and carbon (C) than clayey soils at the soil

surface, a difference that was accentuated by deforestation. Conversely, the higher initial content of Hg in clayey soils was associated with a

more important Hg loss from the soil’s surface. By shedding light on the consequences of family practices for OM, nutrient status and Hg

depletion, this paper gives a new perspective on soil responses to agricultural practices. These conclusions need to be addressed in a strategy

plan to limit family land-use impacts on soils and the surrounding ecosystems. Recommendations for more sustainable land uses are proposed

based on what has been learned about soil responses to local agricultural practices.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Deforestation; Amazon; Agriculture; Land use; Mercury; Cations; Phosphorus; Nitrogen

* Corresponding author at: Biodome de Montreal, 4777 Pierre De Cou-

bertin, Montreal, Quebec, Canada H1V 1B3.

E-mail addresses: [email protected] (N. Farella),

[email protected] (R. Davidson),

[email protected] (M. Lucotte),

[email protected] (S. Daigle).

0167-8809/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.agee.2006.11.003

1. Introduction

In the face of rapid deforestation of the Amazon, growing

concerns call for actions to protect this unique mosaic of

ecosystems. From a conservationist point of view, no reason

may be valuable enough to allow for the destruction of the

most diversified terrestrial ecosystem of the planet.

However, many reasons incite local people to explore

mailto:[email protected]




http://dx.doi.org/10.1016/j.agee.2006.11.003

N. Farella et al. / Agriculture, Ecosystems and Environment 120 (2007) 449–462450

different land-uses of the Amazon region. Economic

incentives are the principal argument put forward in the

debate and deforestation benefits many actors, from the

government to timber companies, from large landowners to

family farmers (Walker et al., 1997; Margulis, 2004).

Depending on the Amazon region considered, deforestation

for agricultural purposes may be for large-scale activities

such as soy plantations and pastures or for subsistence

farming. Family farming is widespread all over the Amazon,

some regions being rapidly deforested as a result. In

Rondonia state, for example, practically all land clearing is

carried out by small-scale cultivators (Calviglia-Harris,

2003). Family farming is also the leading cause of

deforestation, ahead of large-scale pastures and tree

harvesting, in the study region of this research, which is

located in the State of Para near the TransAmazon highway.

Sustainable agriculture is often proposed as a solution to

limit slash-and-burn practices. However, proposed solutions

have to be adapted to local communities, taking into account

their cultural, economic and environmental characteristics.

This study was part of a broader research project, which

aims at better understanding family agriculture in a recently

colonized region. Deforestation is destabilizing the entire

ecosystem and among other impacts, it has been suggested

that Hg is leached from soils, possibly contributing to

aquatic ecosystem contamination (Farella et al., 2001;

Roulet et al., 1998). Since fish Hg contamination poses a

threat to human health (Lebel et al., 1998), addressing the

environmental Hg cycle is crucial. This research project

considered changes in farming practices to lower Hg

contamination, to protect soil and forest resources, and to

ensure a long-term land occupation for concerned popula-

tions. Sociological, economic and geographic aspects were

studied and gave a comprehensive portrait of subsistence

farming near the TransAmazon highway (Farella, 2005).

The specific focus of this paper was to analyze changes in

fertility and Hg retention in soils submitted to family

farming practices through a combination of measurements:

OM, cations, N, P and Hg. Most studies on the responses of

soils submitted to varied land-uses in the Amazon propose a

comparative analysis of these land-uses on sites with a

singular land-use since deforestation (e.g. Cerri et al., 1991;

Holscher et al., 1997; Sanchez et al., 1983; Garcia-Montiel

et al., 2000). However, these studies do not take into account

the complexity and diversity of land-uses on family farms.

The present study aimed to address this knowledge gap by

including sites that were representative of actual family

land-uses. Samples were situated on sites presenting a

variety of historical uses such as numerous cycles of crops

and fallows. With a representative sampling of household

land, this study allowed us to get a more precise perspective

on soil responses to farming practices. The use of medians

while treating thousands of data points circumvents

problems inherent to soil heterogeneity and gives an

accurate portrait of soil response to land use. Ultimately,

the objective of this paper is to propose strategies for the

elaboration of better farming practices for the Tapajos region

and similar regions of the humid tropics, based on measured

soil responses of family farming.

2. Methods

2.1. Research area

The present study took place in a remote region of the

Brazilian Amazon, a few tens of km from the TransAmazon

highway. The nearest city is Itaituba, situated some 60-km

southwest of the study region along the Tapajos River. The

entire region around Itaituba is an active colonization front.

Moreover, the state of Para is one of the three Amazonian

states in which 85% of total Brazilian Amazonian deforesta-

tion occurs (Margulis, 2004). The TransAmazon Highway

acts as a catalyst for deforestation: Chomitz and Thomas

(2000) estimate that 85% of deforestation occurs in a

perimeter of 50 km from roads. In the state of Para, the annual

deforestation rate was appraised at 8200 km2 in 2002–2003

(INPE, 2004). The specific study region was located near the

village of Brasılia Legal, on the opposite bank of the Tapajos

River, an important affluent of the Amazon River. The

sampling area was chosen based on collective research needs.

Sampling was conducted on family agricultural lands situated

within a few km each other. All of the sampling sites were

located on the banks of three lakes belonging to the Tapajos

floodplain: Lago Pereira, Lago Cupu and Lago Bom Intento.

When questioned, people mentioned living in that area from

less than 1 year up to 37 years (Farella, 2005). The median

time since settlement was 14 years (Farella, 2005). The

principal land-use in this region was the cultivation of

temporary crops (manioc: Manihot esculenta Crantz, rice:

Oryza sativa L., bananas: Musa spp. and beans: Phaseolus

vulgaris L.) interrupted with fallow, and generally ending

with pasture. Apart from these cultivation sequences, small

areas were dedicated to fruit tree plantations, a marginal land-

use. Forest still covered approximately 55% of family land,

but forested areas were lost to newly cultivated plots each year

(Farella, 2005).

2.2. Sampling and analyses

Soil sampling was conducted with the collaboration of

local people. Sampling was pursued only on sites where

individuals could recount the history of land-use, in order to

ensure reliability of past land-use. At the time of sampling,

the precise historical background of land-use was asked to

informants, including the number of years of cultivation, the

number of fires, the number of years of fallow, etc. For each

site, a complete chronology of events was constructed. For

most sites, respondents were able to provide a fairly

complete history of land use. In some cases however, doubts

remained on the total number of years spent in crop or

fallow. Generally, these cases concerned some sites that

N. Farella et al. / Agriculture, Ecosystems and Environment 120 (2007) 449–462 451

were deforested by a former landowner. They were left in the

data set because they represented older sites. Verification of

the history of sites was made with the informants, which

contributed to reducing errors. Time spent in crop, fallow

and pasture were calculated for each site. This characteriza-

tion allowed us to compare sites with very different histories.

For example, time spent in pasture may also include some

years spent in crops or in fallow, and time spent in crop may

also include time spent in fallow. This sampling design is

very innovative as it represents the actual land-use.

A total of 25 family farms were sampled, representing

one hundred different sites. At each site, three different cores

were sampled with a percussion sampler, approximately

10 m apart. The first 5 cm under the leaf litter was

considered as representative of the soil surface, while the

20–25 cm horizon and the 50–55 cm horizon were sampled

to evaluate sub-soil surface pedological dynamics. Prior to

any chemical analysis, samples were passed through a 2-mm

mesh, lyophilized and finally homogenized in a stainless

steel grinder. Several physico-chemical variables were

determined. Hg was extracted with HCl and measured by

atomic fluorescence (Pichet et al., 1999). Cations (Ca, Mg,

K) were extracted with BaCl2 and measured by atomic

absorption (Hendershot et al., 1993). Mineral N (NO3 and

NH4) was extracted with KCl 2 M (Maynard and Kalra,

1993) and analyzed by colorimetry (auto-analyzer TRAACS

800). Total C and N were measured on Carlo-Erba NA-1500

analyzer. Oxy-hydroxides of Fe and Al were extracted using

the citrate–dithionate–bicarbonate (cdb) buffer method

(Lucotte and d’Anglejan, 1985), and analyzed by atomic

absorption. The different P compounds (orthophosphate and

organic) were isolated following a sequential extraction and

measured by colorimetry (auto-analyzer TRAACS 800)

(Lucotte and d’Anglejan, 1985). The abbreviations Fe-cdb,

Al-cdb, P-cdb and P-org are used to qualify these different

chemical fractions.

Since the soils sampled displayed a wide variety of colour

and texture, soils were categorized in order to minimize

differences essentially due to the inherent heterogeneity of

soil types. Two approaches were tested to propose a valid

soil discrimination model. First, a colour characterization

based on the Munsell chart was made for each dry soil

sample. Colours showed a wide gradient of tones from

grayish yellow to brownish red. This significant hetero-

geneity of soil colours prevented grouping, hence, a second

method, based on fine particle content (FP), was chosen to

distinguish soil categories. Granulometric fractions were

weighed after humid fractionation and soil groups were

separated following their content of fine particles, defined as

smaller than 63 mm. Fine particle content is recognized to

play an important role in tropical soil chemical properties

(Botschek et al., 1996; Bernoux et al., 1998). A factorial

analysis including all sites and their corresponding

percentile of FP showed that a threshold of 65% FP allowed

for the differentiation of our soil sample pool into two

distinct groups. The soil group with less than 65% FP

corresponded to paler brown-yellowish soils, possibly

related to the group of Ultisols according to the USDA

classification (or Acrisols for FAO/UNESCO). The soil

group characterized by more than 65% FP included mainly

darker brown-yellowish soils and is possibly related to the

Oxisols of the USDA classification (or Ferralsols for FAO/

UNESCO). We will use the terminology clay-sandy soils

(<65% FP) and clayey soils (>65% FP) to label these two

soil groups throughout the present article.

2.3. Soil data set presentation

Most studies illustrating the impacts of land-use through

time on the soils of the Amazon region are based on the

analysis of a few sites and compare forested sites to sites

submitted to one land-use over a certain period of time. In

this study a different approach was used, based on a broad

sampling which allowed us to explore the impacts of non-

linear land-use on soils over a period of a quarter of a

century. The soil-sampling program was pursued on the

agricultural land of 25 households. Each site had a different

historical record; some being deforested for less than a year

while others went through many subsequent cycles of crop

and fallow. To consider this complex historical land use,

thus allowing to compare all sites together, we propose an

innovative approach. Time spent in crop, fallow and pasture

were summed for each site. Figs. 1–10 feature median

values of soil variables (at surface and at 50 cm depth) in

relation to the time spent in a particular land-use. The use of

medians proved to be more stable and accurate than means

because of the skewed distribution of data and the

heterogeneity in soil types. The graphs using medians

display clear trends that allow for reasonable interpreta-

tions. Nine chemical soil variables are presented in this

paper. Twenty other variables were also measured and

principal component analysis were performed on these data

(see Farella et al., 2006). Table 1 shows the time frame

categories established for the soil data set which were used

to build the figures, in relation to the three land-uses

(pasture, crop and fallow) and the two soil types (clay-sandy

and clayey). These categories were determined in order to

obtain a sufficient sample size for each (n > 9 in general)

and to be as balanced as possible within each category.

Categories were also determined in such a way as to obtain

comparable time frames between the two soil types. In each

time frame category, the median value was chosen to

represent this time interval on the x-axis. For instance, a

value of 2.5 years was used to represent the time frame

category 1–4 years. The values on the y-axis correspond to

the median value of all samples within this time interval.

This methodology allowed for a clear graphic presentation

of the soil data given the naturally high soil heterogeneity.

Median soil values were compared to a baseline that

corresponds to the unperturbed forested sites. Twenty-seven

and 15 samples from forest sites were used for clay-sandy

soils and clayey soils, respectively.


Fig. 1. C content in soil in relation with years of pasture, crop and fallow.

(A) Surface of clayey soil; (B) surface of clay-sandy soil; (C) depth of

clayey soil; (D) depth of clay-sandy soil.

Fig. 2. Ca content in soil in relation with years of pasture, crop and fallow.



3. Results

3.1. C content variation in soil

The C content trend was specific for each soil type (Fig. 1).

For clayey soils, nearly all median-values of soils submitted to

a land-use showed a C content superior to the median of

forested soils. At the surface, C content went from 3.3% up to

3.8% while at deeper levels it went from 0.7% to around 0.8%.

However, this increase tended to slow after a certain amount

of time spent under cultivation: after 5 years in crop and 10

years in pasture or fallow, C content in clayey soils tended to


Fig. 3. Mg content in soil in relation with years of pasture, crop and fallow.



Fig. 4. K content in soil in relation with years of pasture, crop and fallow.



fall back to initial values. C content increased again with time

spent in fallow, showing an important C enrichment beyond

15 years. For clay-sandy soils, the picture was quite different:

there was a clear depletion of surface C content which was

more marked with time spent in crop than with time spent in

pasture. Indeed, C content dropped from 2.3% in forested soils

to 1.7% or less after soils had been cultivated for 3 years or

more. In the deepest layers, C content was similar in forested

soils and in soils submitted to any of the three land-uses,

although therewas a slight tendency for enrichment especially

in old fallows. However, a land use superior to 5 years seemed

to contribute for C enrichment in both soil horizons.


Fig. 5. NH4 content in soil in relation with years of pasture, crop and fallow.



Fig. 6. NO3 content in soil in relation with years of pasture, crop and fallow.



3.2. Base cations content variation in soil

Ca, Mg and K (hereafter refered to as ‘‘base cations’’)

showed a significant increase with duration of land-use

(Figs. 2–4). Surface soils tended to experience a greater

enrichment than deeper soils. Of the three base cations, Ca

increased the most, followed by Mg and then K. In absolute

values, all three cations increased at the surface of clayey

soils: the Ca content of forested clayey soils was only

0.2 cmol kg�1 but reached 2–5 cmol kg�1 when submitted

to any of the three land-uses (Fig. 2); Mg started at

0.3 cmol kg�1 in forested clayey soils and reached more


Fig. 7. P-org content in soil in relation with years of pasture, crop and

fallow. (A) Surface of clayey soil; (B) surface of clay-sandy soil; (C) depth

of clayey soil; (D) depth of clay-sandy soil.

Fig. 8. P-cdb content in soil in relation with years of pasture, crop and

fallow. (A) Surface of clayey soil; (B) surface of clay-sandy soil; (C) depth

of clayey soil; (D) depth of clay-sandy soil.

than 1.5 cmol kg�1 through land-use (Fig. 3) and K went

from 0.11 cmol kg�1 in forested clayey soils to over

0.20 cmol kg�1 with land-use (Fig. 4). Base cation content

tended to be correlated with time spent in fallow. Ca content

tended to fall back to forested values after 15 years or more

of fallow. This trend was observed both at the surface and in

deeper levels of both soil types. The same could also be said

of Mg and K content in clayey soils, but generally not in

clay-sandy soils where only Mg tended to return to forest

values at the top layer. In pastures, base cation content

tended to stay enriched even after 10 years of land-use with

the only exception of K content which decreased. As for


Fig. 9. Relationships between P-cdb and Fe, Al-cdb, and between P-org and

C in soil under pasture, crop and fallow.

Fig. 10. Hg content in soil in relation with years of pasture, crop and fallow.



crops established on clayey soils, all base cations decreased

rapidly during the first 5 years spent in crop. There was no

such clear trend for clay-sandy soils: Ca and Mg content

tended to stay above forested values even after 5 years in

crop, while K content returned to pre-perturbation values at

the surface.


Table 1

Classification of the soil data set in relation to time frame categories for graph design

Clay-sandy soils Clayey soils

Time frame categories

(years)

Median time in

land-use (years)

No. of

samples (n)

Time frame

categories (years)

Median time in

land-use (years)

No. of

samples (n)

Time spent in pasturea

1–4 2.5 18 1–4 2.5 15

5–8 6.5 15 5–8 6.5 12

9–13 11 9 9–13 11 9

Time spent in cropa

1 1 48 1 1 36

2 2 54 2 2 21

3 3 12 3 3 15

4 4 9 4–5 4.5 9

5–7 6 9

Time spent in fallowa

1–3 2 36 1–3 2 12

4–6 5 15 4–6 5 9

7–9 8 21 7–9 8 24

10–19 14.5 15 10–19 14.5 12

20–35 27.5 6 20–35 27.5 9

a Soils that supported pasture, crop and fallow were grouped according to the time spent in each specific land-use. In each time frame category, the median

value was chosen to represent this time interval on the x-axis. For instance, a value of 2.5 years was used to represent the time frame category 1–4 years. In the

figures, the median content for the chemical variable stands for these time frame categories.

3.3. N content variation in soils

Available N (NH4 and NO3) was the most significantly

diminished nutrient in all land uses. NH4 and nitrate (NO3)

content of forest soils were nearly always superior to

deforested soils (Figs. 5 and 6). NH4 depletion was more

marked than NO3 depletion. Time spent in fallow, pasture and

crop could cause a three-fold loss of NH4 from surface soils,

dropping from 3 mmol g�1 in the clayey soils of forests to

values between 0.8 and 1.8 mmol g�1 for different land-uses

and from 1.4 mmol g�1 in clay-sandy forest soils to values

between 0.4 and 1.0 mmol g�1 under cultivation (Fig. 5). NO3

depletion was most marked at the surface, especially for

clayey soils, but could also be observed at deeper layers for

clayey soils (Fig. 6). In deeper layers of cultivated land, NO3

and NH4 levels tended to equal and sometimes even exceed

forest soil values. NH4 retention is observed in deeper

horizon, which is more important in clayey soils than in clay-

sandy soils. Differences between land-uses were not striking;

however, some particularities should be highlighted. First,

NO3 and NH4 depletion was noted after only 1 year of

cultivation, suggesting that this impact occurs rapidly after

deforestation. Nonetheless, after 15 years of fallow, NO3 and

NH4 content tended to go back to values found in forest soils.

3.4. P content variation in soil

Of both P forms presented in this study, only organic P (P-

org) showed a clear response to land-use. A distinct P-org

enrichment was observed for clay-sandy soils: the median

content of forest soils went from 3 mmol g�1 at both the

surface and deeper layers to 5 mmol g�1 after some time

spent in crop, pasture or fallow (Fig. 7). For clayey soils,

median forest soil values were higher and an increase was

only observed in sub-surface layers where P-org content

went from 5 mmol g�1 up to 7 mmol g�1 (Fig. 7). In both

types of soil, maximum values were attained beyond 15

years of fallow. Contrary to P-org, orthophosphate (P-cdb)

content in soils did not seem to be clearly influenced by land-

use (Fig. 8). In terms of absolute content, clayey soils

showed median concentrations four times greater than clay-

sandy soils (8 mmol g�1 compared to 2 mmol g�1). More-

over, median values varied much more for clayey soils,

ranging from 2 mmol g�1 to 12 mmol g�1, whereas clay-

sandy soils showed relatively constant values in the study

sites. Fig. 9 shows the relationships between different P

fractions and other soil variables. Both iron and aluminum

oxyhydroxides (Fe-cdb and Al-cdb) are correlated to P-cdb

content (Fig. 9). P-cdb was correlated to the Fe-cdb and Al-

cdb content of soils with a R2 of 0.81 and 0.72, respectively.

However, soil P-org content showed a different trend: P-org

accumulation was strongly correlated to soil C content, with

R2-values above 0.8.

3.5. Hg content variation in soil

Soil Hg content tended to decrease with duration of land-

use, especially at the soil surface, eventually stabilizing to

reach a rather constant minimum level (Fig. 10). In the first

horizon of clay-sandy soils, Hg content went from 70 ppb in

forest soils down to around 55 ppb for both crop and fallow

after 5 years. At the surface of clayey soils, the 130 ppb

median value of Hg in forest sites fell to a threshold value of

around 95 ppb for all land-uses; this drop was recorded after


the first year of crop. At depth, the drop of Hg through time

caused by land-use was less significant. Surface soils lost up

to 35% of the original Hg content while deeper soils showed

a smaller depletion, up to 10%. In absolute terms, the overall

loss for clayey soils was more considerable since the initial

value was larger: an approximate average value of 35 ppb of

Hg was lost from the top layers of clayey soils, while

approximately 15 ppb of Hg was lost from clay-sandy

surface soils.

4. Discussion

4.1. Soil chemistry

Chemical variations in soils were observed when

comparing farmed land to that of forested sites. Soils in

their first crop cycle underwent a Hg depletion similar to that

of soils submitted to long-term land-use, especially in the

case of clayey soils (Fig. 10). This means that Hg depletion

occurs rapidly after deforestation and that simply burning

the forest triggers Hg depletion (also see Farella et al., 2006).

Hg content does not seem to fall beneath a given threshold

indicating that Hg loss attains a limit at a particular soil

depth. Other studies in the Amazon region also proved that

deforestation causes soil Hg depletion. Roulet et al. (1998)

showed that Hg leaching process affects primarily surface

horizon. Almeida et al. (2005) observed that 80 cm soil

profiles under pasture and silviculture have lower Hg content

than soils in forests. Lacerda et al. (2004) measured lower

Hg concentrations in surface soils (until 10 cm) under

pasture compared to soils in forests. Fostier et al. (2000)

clearly demonstrated that deforestation contributes to a

three-fold increase in Hg content of a stream output

compared to a natural area. Thus, each hectare that

undergoes slash-and-burn represents yet another source of

Hg to aquatic ecosystems. The rapid Hg loss after

deforestation observed in this study constitutes a strong

incentive to limit further deforestation.

In an analogous fashion, NH4 and NO3 content also

diminish right after deforestation and, again, depletion is

greater in clayey soils (Figs. 5 and 6). While NO3 depletion

is probably driven by lixiviation, the NH4 decrease could be

related to the competition caused by lasting cations at the

surface, as is the case for Hg. Soil N dynamics in the

Amazonian region is an important issue since it is a limiting

nutrient (Tiessen et al., 1994b). Even without soil

perturbation, Amazonian soils suffer from low N retention:

NO3 is a very mobile nutrient subject to leaching while NH4

is prone to competition with other cations. N is rendered

even more mobile after fires and farming because of several

processes such as the reduction of OM inputs due to the

absence of forest litterfall, stimulation of OM mineralization

resulting from higher soil temperatures and increased

leaching through rainfall (Brown et al., 1994; Roose,

1986). These processes, added to the N uptake by crops and

pasture, might contribute to N depletion after deforestation.

A literature review by Murty et al. (2002), which gathers

results from 61 sites, emphasizes that average N loss with

land-use was 15%. However, not all studies showed this N

depletion with land-use. For example, in Amazonia which

looked at soils similar to those found in our study site, NO3

and NH4 content first increased following slash-and-burn

then subsequently decreased but not below forest values

(Hughes et al., 2002). Other studies in Rondonia reported an

increase in total N in pasture (Moraes et al., 1996) and more

specifically with respect to pasture duration (Feigl et al.,

1995).

Base cation content also responds very rapidly after

deforestation. It has been shown that post-burn ashes contain

around half of the Ca and K originally in tropical forest

biomass (Giardina et al., 2000). Base cation enrichment of

soils upon forest burning has been widely documented (Juo

and Manu, 1996; Sanchez et al., 1983; McGrath et al., 2001).

Such enrichment in base cations was indeed observed in the

soils of our study which had been cropped only 1 year as

well as in soils that had been submitted to pasture for more

than a decade. This fertilizing effect is important for

agricultural purposes given that K, Ca and Mg are often

considered limiting nutrients or at least scarce in Amazonian

soils (Jordan, 1984; Cochrane and Sanchez, 1982; Cuevas

and Medina, 1988; Sollins, 1998). However, this base cation

enrichment is only temporary as illustrated by the Ca content

which tended to diminish when fallow time exceeded 15

years, probably due to cation immobilization in the fallow

biomass. Depletion of Mg and K also occurred in the years

following deforestation, but only in clayey soils. The N

limitation in clay-sandy soils may lead to a smaller fallow

biomass and therefore to a lower uptake of base cations by

successional vegetation. A 20 year chronosequence study in

soils under pasture in Rondonia also showed a cation

increase followed by a gradual decline, this trend is greater

with Ca than with K and Mg (Moraes et al., 1996). Farming

practices executed over a longer time period would probably

contribute to stronger base cation depletion (Juo and Manu,

1996; Cochrane and Sanchez, 1982; Folster, 1986). Decline

of nutrients with land-use is known to depend on many

factors including lack of vegetation cover, numerous

clearings, frequent cultivation sequences, destruction of

the forest root mat and reduction of soil biota (Juo and Manu,

1996).

Likewise, a portion of the P contained in the aboveground

biomass is deposited on the soil surface after forest burning.

The fertilization effect of the ash contributes to the

enhancement of P levels in soils. The quantity of P returned

to soils through ash has been reported to be comparable to

that of base cations, with around half of the P from the

biomass reaching the soil surface (Giardina et al., 2000).

While the P-org pool is enriched upon deforestation, this is

not the case with the P-cdb content. We do not observe any

clear trend in the P-cdb fraction, but the effect of long-term

fallows and pastures on soil P accumulation reflects the


tendency to an increase of the P-org pool. It is noteworthy

that the P-org in our study strictly refers to a recalcitrant P

form extracted with HCl after combustion, while the P-cdb

form of our study is somewhat similar to the Po fraction

reported in various studies (Neufeldt et al., 2000; Garcia-

Montiel et al., 2000; Linquist et al., 1997; Iyamuremye et al.,

1996a,b; Maroko et al., 1999; Ball-Coelho et al., 1993). The

correlations shown in Fig. 9 suggest that OM cycling is

primarily responsible for regulating P-org inputs once land

has been cleared. The clay, Fe and Al-oxides and organic

complexes content of oxisols and ultisols promote fixation

and immobilization of P (Rao et al., 1999; Ayodele and

Agboola, 1981). From our results, it seems that deforestation

and land-use are not affecting the P linked to Fe and Al oxy-

hydroxides, while that P-org is affected by the cycling of

burnt biomass.

There are a few noticeable differences between the soil

responses of crops, pastures and fallow. A period longer than

3 years spent in crop contributes to the maximum soil

depletion in Hg, NH4 and C, as well as to lower the initial

base cation enrichment. This tendency for nutrient loss

through time of cultivation has also been reported by others

(Hughes et al., 2002; Folster, 1986; Sanchez et al., 1983).

Moreover, Tiessen et al. (1994a) emphasize that Amazonian

soils can support cultivation for only 3 years, because of a

naturally low nutrient content, interruption of forest litter

cycling and degradation of the existing pool of OM. Pastures

may be sustained longer on Amazonian soils than crops

because of a lower soil nutrient depletion and higher OM soil

content (Murty et al., 2002; Muller et al., 2004; McGrath

et al., 2001). In our study though, a period longer than 10

years spent in pasture caused an important depletion of NO3

and K content. Serrao and Toledo (1990) stated that

complete soil coverage with pasture can be much more

efficient in reducing soil erosion and maintaining the

physical and chemical properties of soils than frequent

cropping, but that non-managed pastures might present a

high degree of degradation beyond 7 years. The pastures in

our study area were generally poorly managed, thus making

them more prone to soil degradation. Fallows greater than 15

years tend to help soils recover forest level nutrient content

(Figs. 2 and 3). The tendency of fallows to help replenish N

content through atmospheric fixation and recycling from

sub-soils has been demonstrated elsewhere (Maroko et al.,

1998; Schroth et al., 2000). The recovering capacity of

fallows depends on many factors such as length of fallow,

species diversity and inherent soil fertility (Juo and Manu,

1996).

Soil responses are influenced by the initial soil properties

related to texture. Absolute values of Hg depletion are two

times greater for clayey soils. In addition to the higher

natural Hg content of clayey soils, the greater depletion

might also be a direct consequence of greater base cation

enrichment (Farella et al., 2006). Indeed, the clay content of

soils favours cations retention (Sposito, 1989), and

deforested clayey soils retain base cations from the burnt

aboveground biomass more easily. In contrast, clay-sandy

soils retain the new inputs of base cations less efficiently,

leading to less Hg leaching. The maximum loss of Hg for

clay-sandy soils was recorded in soils that underwent 3 or 4

years of cropping whereas this loss was more rapid for

clayey soils (Fig. 10). The build-up of the P-org pool also

responds differently in clayey and clay-sandy soils, with the

latter showing a greater increase (Figs. 8 and 9). Forests that

grow on clay-sandy soils often develop a denser root mat as

well as a deeper root system than soils with higher clay

content. The slow degradation of this belowground biomass

possibly contributes to enhance the P-org pool in clay-sandy

soils. Similarly, gradual degradation of OM from unburned

slashed biomass was reported as a source of slow release of P

in soils (Garcia-Montiel et al., 2000). However, even with

these new soil P sources emanating from slash-and-burn

practices, the absolute content of P-org for clayey soils still

remains higher than for clay-sandy soils, as was the case for

base cations.

4.2. Recommendations for better family farming

Two main incentives drive the call for a more sustainable

agriculture in the Amazon region: (1) reduction of soil

nutrient depletion in order to allow for long-term land-use

and lower deforestation rate; (2) reduction of Hg contam-

ination of aquatic ecosystems and subsequent harm to

populations dependant on aquatic resources.

4.2.1. Prioritizing cultivation on clayey soils

Cultivation of clay-sandy soils does not appear to be

sustainable, due to their inferior fertility and a more dramatic

nutrient loss upon deforestation than clayey soils, especially

for available N. Any further N loss would be critical given

the existing NO3 and NH4 limitations. Pasture management

that would favour N fixation (Rao et al., 1999) or at least that

would not cause any N depletion (Muller et al., 2004;

McGrath et al., 2001; Neill et al., 1995) would therefore be

more acceptable than nutrient-demanding crops. The lack of

any positive impact of pastures on N content in our data set is

probably a consequence of poor management. Considering

base cation soil content, again, farming practices appear to

be less sustainable on clay-sandy soils according to the

fertility index proposed by Cochrane and Sanchez (1982).

Both Mg and K contents are often under the limit suggested

for infertile soils for clay-sandy soils submitted to land-use.

Therefore, new deforestation for farming purposes should be

restricted to clayey soils, which can support longer and more

complex cultivation sequences, helping to reduce the

deforestation rate. Prioritizing cultivation on clayey soils

would probably be acceptable from the farmers’ perspective,

since 70% of them prefer planting on clayey soils (Farella,

2005). Favouring clayey soils would also reduce soil erosion

from slopes since they are frequently found on plateaus

(Roulet et al., 1998). However, clayey soils undergo greater

Hg leaching as a result of their high natural Hg and also the


added impact of base cation inputs. Some strategies would

be necessary to reduce this adverse effect upon cultivation,

like the use of trees, whether inter-planted with crops or left

as a border around cultivated areas. Moreover, since three

quarters of the study population prefers to plant rice in newly

deforested areas, this crop could be reduced to favour

manioc, beans and corn (Zea mays L.), usually planted in

burnt fallow (Farella, 2005).

4.2.2. Favouring long fallow

Longer fallows should be encouraged because of their

numerous beneficial impacts especially as to a marked

reduction of soil erosion. However, longer fallow might not be

easily integrated into family farming practices. Presently, the

most common fallow duration is 3 years and farmers agree

that this is a sufficient length to allow for crop production that

covers the household needs (Farella, 2005). Given that the

positive effects of long fallowing periods are not well

understood, environmental education might be useful to

promote them. Managed fallows are an alternative proposed

by researchers to ensure longer term soil fertility and to

optimize soil land-use, contributing to reduce deforestation

rates. For example, fallows enhanced with certain trees or

legumes have proven to replenish soil N (Maroko et al., 1998)

and to enhance subsequent crop production (Szott et al.,

1994). More research is needed to identify appropriate fallow

management strategies for specific regions. Factors such as

species selection, biomass production, nutrient sequestration,

N and P availability, weed competition, mycorrhiza infec-

tions, etc., need to be worked out. Hedden-Dunkhorst et al.

(2003) demonstrate how fallow vegetation acts as a safety net

through enhancing products extraction and thus household

incomes and through providing environmental services by

increasing soil fertility and flora diversity. Hence, these

authors laud the fallow ecosystem for the beneficial role it can

play in environmental and socio-economic sustainability. Our

results show the soil beneficial impacts of fallows, where trees

predominate. In that sense, literature relates various spatial

and temporal associations of trees and crops to allow for more

sustainable agricultural practices (Szott et al., 1991; Dubois

et al., 1996; Leakey, 1999; Rao et al., 1998; Browder and

Pedlowski, 2000).

4.2.3. Promoting soil enrichment

In order to favour long-term cultivation and diminish the

need for further deforestation, alternatives based on nutrient

soil enrichment should also be envisaged. Inorganic

fertilization has been studied widely; however, K leaching

and P fixation remain problematic (Sanchez et al., 1983;

Schroth et al., 2000; Rao et al., 1999). Moreover, the high cost

of inorganic fertilizers prevents them from being used by local

populations. Organic fertilization could possibly overcome

these problems (Ball-Coelho et al., 1993; Iyamuremye et al.,

1996a,b). A research option lies in organic soil enrichment

used with success by indigenous population for centuries,

which have contributed to the formation of fertile soils called

‘‘Terra Preta do Indio’’ (Glaser et al., 2001). Cues could also

be taken from regional farming practices of the Ecuadorian

Amazon where the forest biomass is left to decompose after

felling (slash-and-rot) (Mainville et al., 2006). This strategy

would need to be tested in Central Amazonia where

precipitation is more seasonal. A research-intervention

project in Eastern Amazonia proposes shredding fallow

biomass, then leaving it to decompose on the ground instead

of burning it (chop-and-mulch) (Sommer et al., 2004).

Nutrients in soils have proven to be greatly enhanced after this

treatment and a financial analysis revealed that this process

may double farm incomes (Denich et al., 2004).

5. Conclusion

This paper sheds new light on farming practices and how

they relate to Hg leaching and nutrient content in soils.

Immediate losses of Hg upon deforestation added to the

further progressive loss of N and base cations well illustrate

that cropping and pasture have negative impacts on soils.

Moreover, there is no indication that fruit tree or banana

plantations are better choices in regards to Hg and N loss

(Farella et al., 2006). The act of deforestation per se has the

strongest impact on soils: indeed, no subsequent land-use

can counterbalance the initial Hg loss. These results are of

paramount importance for the planning of sustainable

farming strategies. An integrated approach which aims to

optimize soil fertility and consequently reduce the need for

further deforestation should be developed through encoura-

ging cultivation on clayey soils, favouring longer fallows

and integrating organic fertilization. The next step is more

complex and calls for an alternative to slash-and-burn, as

this deforestation method is conducive to Hg leaching.

Techniques based on leaving the slashed biomass on the

ground could be promising, especially if practiced during the

rainy season. This would help to reduce the sudden release

of Hg and allow for a more gradual transfer of cations from

the slashed forest biomass into the soil for cultivation

purposes. But the fact remains that there are already large

areas of deforested lands for which more sustainable

cultivation practices should be considered based on an

intensification of agriculture. Hundreds of thousands of

families are presently living from subsistence agriculture,

this population growing yearly. Any proposed strategy

would have to be discussed with households to verify their

appropriateness and chances of success. Sustainable land

use on family farms of the Brazilian Amazon needs to be

addressed in an integrated manner, taking into account both

environmental as well as socio-economic aspects.

Acknowledgments

Special thanks to Marie-Claude Bujold, Caroline Dufour,

Andre Dupre, Isabelle Guisset, Etienne Grandmont, Isabelle


Rheault, Sophie Tran and Claire Vasseur, for chemical

analysis; Jean Carreau, Pascale Bernard, Odonaldo Cardoso

Junior, Otavio do Canto Lopes, Lıgia Costa de Sousa, Hugo

Poirier, Sergio Lemos de Oliveira, Regina de Sousa Lima,

Delaine Sampaio, Getulio Walfredo Ferreira Vasques, Joabe

Ramos, for the fieldwork. We are very grateful to the

population of Lago Pereira, Lago Cupu, Lago Bom Intento

and Lago Araipa for their collaborative support. Boat staff

was most helpful in fieldwork support. Thanks to Jena Webb

and Sidney Ribaux for the manuscript revision and to the

anonymous reviewers for their advice. This research was

possible with the financial support of NSERC, FCAR and

IDRC.

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